The complement protein C3 is a common denominator in the activation of the classical, alternative, and lectin pathways of the complement system. In addition, its interactions with several cell-surface receptors make it a key participant in phagocytic and immunoregulatory processes, while its interactions with proteins from foreign pathogens may provide a mechanism by which these microorganisms evade complement neutralization. To date, C3 is known to interact with at least 25 different proteins, including serum proteins, cell surface receptors and proteins of foreign origin. Elucidation of the molecular features related to these C3 associated interactions still requires further analysis of the C3 protein. The long-term goal of this proposal is to identify the structural determinants associated with various functions of this important macromolecule. The proposed study will involve a detailed structural and functional analysis of the interactions between the human C3 fragments C3(H2O), C3b, iC3b, C3c and C3dg, and the complement regulatory proteins, factor H and complement receptor type one (CR1) and to compare these interactions to those of the viral complement regulatory proteins VCP and SPICE with the same C3 fragments. Protein expression, isothermal titration calorimetry (ITC), and hydrogen exchange/mass spectrometry (HDX-MS) will be used to assess the molecular forces that impart specificity and recognition to the interactions between the host complement components and the host and viral regulatory proteins. The functional sites of the interacting molecules will be mapped using HDX-MS, computational methods, and site-directed mutagenesis. The proposed studies are designed to provide basic information on the structural features of C3-ligand interactions as they relate to complement functions. These studies should provide insight into how the virus's complement regulatory molecules help the virus evade complement attack and also assist in the design of specific inhibitors that may have important medical applications.